As indicated above, the natural surjection \(\alpha \)  in a functor
presentation \((\Sigma ,\alpha )\)  can be replaced with a set of flat
equations over \(\Sigma \) , where an equation is called flat if
both sides consist of an operation symbol applied to
variables [1]}. Incidentally, this (standard) term
should not be confused with the same term introduced in the context
of our expression language in Section .
